What does the scratch test tell us?

The Scratch Test in Allergy Diagnosis

The scratch test is a form of skin testing that detects IgE-mediated allergic sensitization but is now rarely used in clinical practice due to its reduced sensitivity, specificity, poor reproducibility, and greater patient discomfort compared to more modern testing methods.

Understanding Scratch Testing

The scratch test is one of the oldest forms of allergy testing that works by:

• Introducing a specific allergen into the patient's skin through a scratched surface
• Allowing the allergen to interact with IgE antibodies on mast cells
• Triggering the release of histamine and other chemical mediators
• Producing a visible wheal and flare reaction within 15-20 minutes in sensitized individuals 1

Limitations of Scratch Testing

Modern clinical practice has largely abandoned scratch testing in favor of more reliable methods because:

• It has reduced sensitivity and specificity compared to skin prick testing 1
• Results show poor reproducibility between tests 1
• It causes greater patient discomfort than alternative methods 1
• Standardization is difficult, leading to inconsistent results 2

Superior Alternatives

Current guidelines recommend:

1. Skin Prick Testing: The preferred first-line test with:

   • Higher sensitivity and specificity (typically over 80% for both) 1
   • Better reproducibility
   • Less patient discomfort
   • More standardized methodology

2. Intradermal Testing: Used when:

   • Prick tests are negative but clinical suspicion remains high
   • Increased sensitivity is required 1
   • Testing for specific allergens that may require deeper penetration

3. Patch Testing: The gold standard for diagnosing allergic contact dermatitis:

   • Involves application of allergens to unaffected skin for 48 hours
   • Readings taken at removal and again at day 4 (sometimes day 7)
   • Detects delayed hypersensitivity reactions 1, 3

Clinical Relevance of Test Results

The British Association of Dermatologists classifies the clinical relevance of positive allergic reactions as:

1. Current relevance: Patient exposed during current episode with improvement upon cessation
2. Past relevance: Previous episode from exposure to allergen
3. Relevance not known: Unclear if exposure is current or past
4. Cross-reaction: Positive test due to cross-reaction with another allergen
5. Exposed: History of exposure without dermatitis or positive test without known exposure 1

Important Considerations

• Antihistamines should be avoided before testing for immediate hypersensitivity reactions 1
• Testing should be deferred after UV exposure (6 weeks), systemic agents (3 months), or biological agents (6 months) 1
• Readings should be performed in natural daylight and rated as positive, negative, or irritant 1
• Some allergens (metals, fragrances) are more likely to cause irritant reactions that can be misinterpreted 1

Historical Context

While research from 1950 suggested that raw foods used in scratch testing could produce larger and more numerous positive reactions than commercial extracts 4, modern standardized testing methods have superseded this approach. A 1994 study demonstrated that immunotherapy based on scratch testing was ineffective in providing symptom relief compared to treatment based on skin end-point titration 2.

In summary, while the scratch test can detect allergic sensitization, its clinical utility has been surpassed by more reliable, reproducible, and comfortable testing methods that provide better guidance for diagnosis and treatment.